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Previously, you could explicitly assign None to a non-null ForeignKey
(or other) field when creating the model (Child(parent=None), etc). We
now throw an exception when you do that, which matches the behaviour
when you assign None to the attribute after creation.
Thanks to ales.zoulek@gmail.com and ondrej.kohout@gmail.com for some
analysis of this problem.
Backport of r9983 from trunk.
git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9984 bcc190cf-cafb-0310-a4f2-bffc1f526a37